It is known that isobutylene and n-butene are separately subjected to the vapor phase ammoxidation for synthesizing methacrylonitrile and vapor phase oxidative dehydrogenation for synthesizing 1,3-butadiene, respectively. The former case is described, for example, in Japanese Pat. Nos. 1613/66, 6897/66, 7771/66, 7854/66, 7856/66, 12731/66, 14093/66, 16778/66, 22476/67, 6045/68, 26288/68, 4092/69 and 28491/69, and the latter case is disclosed typically in Japanese Pat. No. 26842/68. However, no report has been known hitherto, as to the simultaneous preparation of methacrylonitrile and 1,3-butadiene by the vapor phase catalytic ammoxidation and oxidative dehydrogenation of mixed butane-butene.
Recently, the production of monomeric ethylene, which is one of the most important petrochemical starting materials, has been carried out in the so-called naphtha cracking center of the petrochemical complex, with the C.sub.4 B-B fraction (containing the above-described mixed butane-butene together with butadiene) being formed as a by-product in large amounts. This C.sub.4 fraction has heretofore been of little value except for the butadiene and used only as a fuel gas.
With a view to effectively utilizing individual components contained in the C.sub.4 fraction, studies have been made individually for synthesis of methacrylonitrile by ammoxidation of isobutylene and for synthesis of 1,3-butadiene by oxidative dehydrogenation of n-butene and, as a result, several patents have been reported.
The processes of these patents require as starting gaseous material isobutylene and n-butene of high purity. Whereas, the constituents of the mixed butane-butene obtained as the B-B fraction, i.e. isobutane, n-butane, isobutylene, 1-butene, wis-2-butene and trans-2-butene are very similar to one another in their physical and chemical properties. Accordingly, separation and purification of these constituents are difficult and make it fairly expensive to produce starting materials of high purity.
Judging from the aspect of starting materials, it is apparent that the present process is advantageous wherein the mixed butane-butene is directly subjected as such to ammoxidation and oxidative dehydrogenation to yield simultaneously methacrylonitrile and 1,3-butadiene.
Thus, the present invention provides a novel and commercially advantageous process which enables the use of fairly low cost starting material, i.e. the C.sub.4 fraction formed as by-product on cracking of naphtha, or a residuum obtained after extraction of 1,3-butadiene from said fraction (i.e. the mixed butane-butene).
The two products, methacrylonitrile and 1,3-butadiene, are quite different in chemical properties as the former is a nitrile compound and the latter a diolefin. They are different also in physical properties since methacrylonitrile boils at 90.3.degree. C. and 1,3-butadiene at -4.41.degree. C. This makes it possible to effect separation and purification of these compounds very easily by ordinary distillation, resulting in reduction of cost for the production of useful starting chemicals.
According to the present invention, it has been found that although ammoxidation of isobutylene alone produces methacrylonitrile in a single pass yield of 64.8%, the single pass yield of methacrylonitrile is increased to 75.1% by adding n-butene gas to the reaction system. It has also been found that addition of n-butene results in formation of butadiene and its single pass yield and selectivity are approximately as high as those obtained in oxidative dehydrogenation of n-butene alone.
Although it is not clear why the yield of methacrylonitrile is increased when the ammoxidation and oxidative dehydrogenation are carried out simultaneously using the mixed butene-butane as described above, it is thought that competition between the ammoxidation of isobutylene and oxidative dehydrogenation of n-butene may occur and this competition may decrease the concentration of active sites on the surface of the catalyst for the ammoxidation of isobutylene, and/or strongly active sites on the catalyst may be used for the oxidative dehydrogenation of n-butene, thereby avoiding excessive proceeding of the ammoxidation of isobutylene and resulting in increase in selectivity to methacrylonitrile.
In the catalyst used in the present invention, the numbers of individual atoms is preferably within the following ranges:
a: 3-10, b: 1-5, c: 0.6-2, d: 0-3, h: 0.04-0.8, f: 12 and g: 42-68.
The catalyst utilizable in the present invention can be prepared by adding to an aqueous solution of an appropriate molybdate such as ammonium molybdate, at least one of potassium, rubidium, and cesium compounds and then water-soluble iron, bismuth, cobalt, and magnesium compounds, adding, if necessary, a carrier to the resulting slurried suspension, evaporating the mixture to dryness, and treating the resulting cake at high temperatures ranging from 550.degree. to 750.degree. C for 4 hours in the presence of air or oxygen.
Used as the above-described potassium, rubidium, cesium, iron, bismuth, cobalt and magnesium compounds are, for example, nitrates of these metals.
The catalyst may be used as such, i.e. without any carrier to give an excellent yield, although, from the standpoint of catalyst strength, it is preferred to use a small amount of a carrier. Examples of such carriers include inert substances such as silica, silicon carbide and 2-alumina, although the silica is particularly preferred.
The catalyst may be used in the form of granules or tablets.
Although the catalyst may be used in a fixed bed, it is in general desired to use the catalyst in a fluidized or moving bed since the reaction is extremely exothermic.
As molecular oxygen used in the present invention, air is normally employed, although any oxygen-containing gases diluted with an inert gas, e.g. nitrogen which does not affect the desired reaction, may also be used.
Reaction temperatures adopted in the present process may preferably be within the range of from 300.degree. to 500.degree. C and more preferably from 350.degree. to 480.degree. C. The process may be carried out under either superatmospheric or subatmospheric pressure, although it is convenient to conduct the process under normal pressure.
Under real pressure and reaction temperature, contact time of a gaseous mixture consisting of the mixed butane-butene, ammonia and air with the catalyst, is within the range of 0.5-8 seconds, preferably 2-5 seconds.
The mixed gas to be passed through the catalyst is preferably composed of 1-5 moles of oxygen in the form of air and 1-5 moles of ammonia, per mole of effective olefin (i.e. isobutylene plus n-butene) in the mixed butane-butene, and more preferably 1-3 moles of oxygen in the form of air and 1-3 moles of ammonia per mole of the effective olefin. As the desired reaction is exothermal, it is preferred to add 1-30 moles of water in the form of steam.